Making use of two orthogonally polarized signals simultaneously can double the capacity of a radio communications system. In practice, the two orthogonally polarized signals leaving the transmitter are either two orthogonal linearly polarized waves or two opposite circularly polarized waves. During transmission, the orthogonality of the waves may be degraded, or for various reasons cross-talk between channels may occur. Whatever the cause of polarization distortion, the failure to maintain orthogonality will produce two nonorthogonal elliptically polarized waves at the receiving terminal.
Orthogonality can be recovered at the receiving terminal by transforming the two nonorthogonal elliptically polarized waves into two orthogonal linear polarizations. Present art accomplishes this transformation by employing a differential phase shifter and a differential attenuator. This tranformation current takes place in the antenna waveguide by employing mechanical phase shifters and rotary joints. As such, the orthogonality obtained is limited to approximately 10 dB.
The present invention makes it possible to obtain orthogonality approaching 50 dB. The transformation process is fully automatic and will adapt substantially as quickly as the transmitting medium changes. This invention applies the technology of null steerers and adaptive antenna systems to achieve the improved performance.